Increased growth of algae occurs frequently in natural and artificially created bodies of water. The reason is mostly the increased concentration of plant nutrients, in particular nitrogen and phosphorus. That is why considerable efforts are currently made to keep these nutrients away from waters.
Despite the low concentration of said nutrients one can still observe a strong development of algae in many waters. For example, the optimal phosphorus concentration for the growth of the alga Asterionella formosa is 2 .mu.g/l. This value is so low that it can hardly be achieved in bodies of water.
The growth of aquatic plants which could obstruct the growth of algae is usually very low in algae-rich waters. Moreover, a high pH-value can nearly always be observed in the water. Algae encounter particularly favourable living conditions at a high pH-value, whereas aquatic plants which live in nutritive competition with the algae prefer lower pH-values. In order to improve the living conditions for the aquatic plants and, at the same time, to deteriorate those for algae, efforts are also made to lower the pH-value in the water. This is usually performed by adding mineral acids such as hydrochloric acid or sulphuric acid. The addition of these acids, however, is problematic, since they represent, on the one hand, a serious burden to the environment and their application, on the other hand, requires considerable amounts of material and is costly.
It has already been proposed in WO 93/21115 to improve the water quality of stagnant or slow-flowing waters by introducing CO.sub.2 in order to set the pH-value of a body of water to such a range that the growth of algae is prevented. It is generally proposed to introduce technically produced CO.sub.2 gas into the body of water by means of suitable apparatuses such as atomizers or nozzles and in suitable form such as mixed with air or in an aqueous solution. This publication, however, does not disclose a method that would allow providing the required quantities of CO.sub.2 with an acceptable or even low effort.